1. Field of the Invention
The present invention relates to a variable magnification optical system and an image pickup apparatus using the same and, more specifically, to an optical arrangement suitable for use in a video camera, a still video camera, a copying machine and the like which are arranged to realize variation of magnification by using an optical unit having decentered reflecting surfaces, as a magnification varying optical unit.
2. Description of Related Art
It is known that an optical system of the type which is composed of only refracting lenses has been provided as a variable magnification optical system. In such a conventional optical system, refracting lenses each having a spheric surface or aspheric surface of rotational symmetry are rotationally symmetrically arranged with respect to the optical axis.
In addition, various photographing optical systems using reflecting surfaces such as concave mirrors or convex mirrors have heretofore been proposed, and an optical system using both a reflecting system and a refracting system is also well known as a catadioptric system.
FIG. 37 is a schematic view of a so-called mirror optical system which is composed of one concave mirror and one convex mirror. In the mirror optical system shown in FIG. 37, an object light beam 104 from an object is reflected by a concave mirror 101 and travels toward an object side while being converged, and after having been reflected by a convex mirror 102 and having been refracted by a lens 110, the object light beam 104 forms an image of the object on an image plane 103.
This mirror optical system is based on the construction of a so-called Cassegrainian reflecting telescope, and is intended to reduce the entire length of the optical system by bending, by using the two opposed reflecting mirrors, the optical path of a telephoto lens system which is composed of refracting lenses and has an entire large length.
For similar reasons, in the field of an objective lens system which constitutes part of a telescope as well, in addition to the Cassegrainian type, various other types which are arranged to reduce the entire length of an optical system by using a plurality of reflecting mirrors have been known.
As is apparent from the above description, it has heretofore been proposed to provide a compact mirror optical system by efficiently bending an optical path by using reflecting mirrors in place of lenses which are commonly used in a photographing lens whose entire length is large.
However, in general, the mirror optical system, such as the Cassegrainian reflecting telescope, has the problem that part of an object ray is blocked by the convex mirror 102. This problem is due to the fact that the convex mirror 102 is placed in the area through which the object light beam 104 passes.
To solve the problem, it has been proposed to provide a mirror optical system which employs decentered reflecting mirrors to prevent a portion of the optical system from blocking the area through which the object light beam 104 passes, i.e., to separate a principal ray of the object light beam 104 from an optical axis 105.
FIG. 38 is a schematic view of the mirror optical system disclosed in U.S. Pat. No. 3,674,334. This mirror optical system solves the above-described blocking problem by using part of reflecting mirrors which are rotationally symmetrical about the optical axis.
In the mirror optical system shown in FIG. 38, a concave mirror 111, a convex mirror 113 and a concave mirror 112 are arranged in the order of passage of the light beam, and these mirrors 111, 113 and 112 are reflecting mirrors which are rotationally symmetrical about an optical axis 114, as shown by two-dot chain lines in FIG. 38. In the shown mirror optical system, a principal ray 116 of an object light beam 115 is separated from the optical axis 114 to prevent shading of the object light beam 115, by using only the upper portion of the concave mirror 111 which is above the optical axis 114 as viewed in FIG. 38, only the lower portion of the convex mirror 113 which is below the optical axis 114 as viewed in FIG. 38, and only the lower portion of the concave mirror 112 which is below the optical axis 114 as viewed in FIG. 38.
FIG. 39 is a schematic view of the mirror optical system disclosed in U.S. Pat. No. 5,063,586. The shown mirror optical system solves the above-described problem by decentering the central axis of each reflecting mirror from an optical axis and separating the principal ray of an object light beam from the optical axis. As shown in FIG. 39 in which an axis perpendicular to an object plane 121 is defined as an optical axis 127, a convex mirror 122, a concave mirror 123, a convex mirror 124 and a concave mirror 125 are arranged in the order of passage of the light beam, and the central coordinates and central axes 122a, 123a, 124a and 125a (axes which respectively connect the centers of reflecting surfaces and the centers of curvature thereof) of the reflecting surfaces of the respective mirrors 122 to 125 are decentered from the optical axis 127. In the shown mirror optical system, by appropriately setting the amount of decentering and the radius of curvature of each of the surfaces, each of the reflecting mirrors is prevented from shading an object light beam 128, so that an object image is efficiently formed on an image plane 126.
In addition, U.S. Pat. Nos. 4,737,021 and 4,265,510 also disclose an arrangement for preventing the shading problem by using part of a reflecting mirror which is rotationally symmetrical about an optical axis, or an arrangement for preventing the shading problem by decentering the central axis of the reflecting mirror from the optical axis.
One example of a catadioptric optical system which uses both a reflecting mirror and a refracting lens and has a magnification varying function is a deep-sky telescope such as that disclosed in each of U.S. Pat. Nos. 4,477,156 and 4,571,036. The deep-sky telescope uses a parabolic reflecting mirror as a primary mirror and has a magnification varying function using an Erfle eyepiece.
Another variable magnification optical system is known which varies the image forming magnification (focal length) of a photographing optical system by relatively moving a plurality of reflecting mirrors which constitute part of the aforesaid type of mirror optical system.
For example, U.S. Pat. No. 4,812,030 discloses an art for performing variation of the magnification of the photographing optical system by relatively varying the distance between the concave mirror 101 and the convex mirror 102 and the distance between the convex mirror 102 and the image plane 103 in the construction of the Cassegrainian reflecting telescope shown in FIG. 37.
FIG. 40 is a schematic view of another embodiment disclosed in U.S. Pat. No. 4,812,030. In the shown embodiment, an object light beam 138 from an object is made incident on and reflected by a first concave mirror 131, and travels toward an object side as a converging light beam and is made incident on a first convex mirror 132. The light beam is reflected toward an image forming plane by the first convex mirror 132 and is made incident on a second convex mirror 134 as an approximately parallel light beam. The light beam is reflected by the second convex mirror 134 and is made incident on a second concave mirror 135 as a diverging light beam. The light beam is reflected by the second concave mirror 135 as a converging light beam and forms an image of the object on an image plane 137. In this arrangement, by varying the distance between the first concave mirror 131 and the first convex mirror 132 and the distance between the second convex mirror 134 and the second concave mirror 135, zooming is performed and the focal length of the entire mirror optical system is varied.
In the arrangement disclosed in U.S. Pat. No. 4,993,818, an image formed by the Cassegrainian reflecting telescope shown in FIG. 37 is secondarily formed by another mirror optical system provided in a rear stage, and the magnification of the entire photographing optical system is varied by varying the image forming magnification of that secondary image forming mirror optical system.
In any of the above-described reflecting types of photographing optical systems, a large number of constituent components are needed and individual optical components need to be assembled with high accuracy to obtain the required optical performance. Particularly since the relative position accuracy of each of the reflecting mirrors is strict, it is indispensable to adjust the position and the angle of each of the reflecting mirrors.
One proposed approach to solving this problem is to eliminate the incorporation error of optical components which occurs during assembly, as by forming a mirror system as one block.
A conventional example in which a multiplicity of reflecting surfaces are formed as one block is an optical prism, such as a pentagonal roof prism or a Porro prism, which is used in, for example, a viewfinder optical system. In the case of such a prism, since a plurality of reflecting surfaces are integrally formed, the relative positional relationships between the respective reflecting surfaces are set with high accuracy, so that adjustment of the relative positions between the respective reflecting surfaces is not needed. Incidentally, the primary function of the prism is to reverse an image by varying the direction in which a ray travels, and each of the reflecting surfaces consists of a plane surface.
Another type of optical system, such as a prism having reflecting surfaces with curvatures, is also known.
FIG. 41 is a schematic view of the essential portion of the observing optical system which is disclosed in U.S. Pat. No. 4,775,217. This observing optical system is an optical system which not only allows an observer to observe a scene of the outside but also allows the observer to observe a display image displayed on an information display part, in the form of an image which overlaps the scene.
In this observing optical system, a display light beam 145 which exits from the display image displayed on an information display part 141 is reflected by a surface 142 and travels toward an object side and is made incident on a half-mirror surface 143 consisting of a concave surface. After having been reflected by the half-mirror surface 143, the display light beam 145 is formed into an approximately parallel light beam by the refractive power of the half-mirror surface 143. This approximately parallel light beam is refracted by and passes through a surface 142, and forms a magnified virtual image of the display image and enters a pupil 144 of an observer so that the observer recognizes the display image.
In the meantime, an object light beam 146 from an object is incidence on a surface 147 which is approximately parallel to the reflecting surface 142, and is then refracted by the surface 147 and reaches the half-mirror surface 143 which is a concave surface. Since the concave surface 143 is coated with an evaporated semi-transparent film, part of the object light beam 146 passes through the concave surface 143, is refracted by and passes through the surface 142, and enters the pupil 144 of the observer. Thus, the observer can visually recognize the display image as an image which overlaps the scene of the outside.
FIG. 42 is a schematic view of the essential portion of the observing optical system disclosed in Japanese Laid-Open Patent Application No. Hei 2-297516. This observing optical system is also an optical system which not only allows an observer to observe a scene of the outside but also allows the observer to observe a display image displayed on an information display part, as an image which overlaps the scene.
In this observing optical system, a display light beam 154 which exits from an information display part 150 passes through a plane surface 157 which constitutes part of a prism Pa, and is made incident on a parabolic reflecting surf ace 151. The display light beam 154 is reflected by the reflecting surface 151 as a converging light beam, and forms an image on a focal plane 156. At this time, the display light beam 154 reflected by the reflecting surface 151 reaches the focal plane 156 while being totally reflected between two parallel plane surfaces 157 and 158 which constitute part of the prism Pa. Thus, the thinning of the entire optical system is achieved.
Then, the display light beam 154 which exits from the focal plane 156 as a diverging light beam is totally reflected between the plane surface 157 and the plane surface 158, and is made incident on a half-mirror surface 152 which consists of a parabolic surface. The display light beam 154 is reflected by the half-mirror surface 152 and, at the same time, not only is a magnified virtual image of a display image formed but also the display light beam 154 is formed into an approximately parallel light beam by the refractive power of the half-mirror surface 152. The obtained light beam passes through the surface 157 and enters a pupil 153 of the observer, so that the observer can recognize the display image. in the meantime, an object light beam 155 from the outside passes through a surface 158b which constitutes part of a prism Pb, then through the half-mirror surface 152 which consists of a parabolic surface, then through the surface 157, and is then made incident on the pupil 153 of the observer. Thus, the observer visually recognizes the display image as an image which overlaps the scene of the outside.
As another example which uses an optical element on a reflecting surface of a prism, optical heads for optical pickups are disclosed in, for example, Japanese Laid-Open Patent Application Nos. Hei 5-12704 and Hei 6-139612. In these optical heads, after the light outputted from a semiconductor laser has been reflected by a Fresnel surface or a hologram surface, the reflected light is focused on a surface of a disk and the light reflected from the disk is conducted to a detector.
However, in any of the aforesaid optical systems composed of conventional refracting optical elements only, a stop is disposed in the inside of the optical system, and an entrance pupil is in many cases formed at a position deep in the optical system. This leads to the problem that the larger the distance to a pupil plane lying at a position which is the closest to the object side as viewed from the stop, the effective ray diameter of the entrance pupil becomes the larger with the enlargement of the angle of view.
In any of the above-described mirror optical systems having the decentered mirrors, which are disclosed in U.S. Pat. Nos. 3,674,334, 5,063,586 and 4,265,510, since the individual reflecting mirrors are disposed with different amounts of decentering, the mounting structure of each of the reflecting mirrors is very complicated and the mounting accuracy of the reflecting mirrors is very difficult to ensure.
In either of the above-described photographing optical systems having the magnification varying functions, which are disclosed in U.S. Pat. Nos. 4,812,030 and 4,993,818, since a large number of constituent components, such as a reflecting mirror or an image forming lens, are needed, it is necessary to assemble each optical part with high accuracy to realize the required optical performance.
In particular, since the relative position accuracy of the reflecting mirrors is strict, it is necessary to adjust the position and the angle of each of the reflecting mirrors.
As is known, conventional reflecting types of photographing optical systems have constructions which are suited to a so-called telephoto lens using an optical system having an entire large length and a small angle of view. However, if a photographing optical system which needs fields of view from a standard angle of view to a wide angle of view is to be obtained, the number of reflecting surfaces which are required for aberration correction must be increased, so that a far higher component accuracy and assembly accuracy are needed and the cost and the entire size of the optical system tend to increase.
The above-described observing optical system disclosed in U.S. Pat. No. 4,775,217 is realized as a small-sized observing optical system which is composed of a plane refracting surface and a concave half-mirror surface. However, the exit surface 142 for the respective light beams 145 and 146 from the information display part 141 and the outside needs to be used as a total reflecting surface for the light beam 145 exiting from the information display part 141, so that it is difficult to give a curvature to the surface 142 and no aberration correction is effected at the exit surface 142.
The above-described observing optical system disclosed in Japanese Laid-Open Patent Application No. Hei 2-297516 is realized as a small-sized observing optical system which is composed of a plane refracting surface, a parabolic reflecting surface and a half-mirror consisting of a parabolic surface. In this observing optical system, the entrance surface 158 and the exit surface 157 for the object light beam 155 from the outside are formed to extend so that their respective extending surfaces can be used as total reflecting surfaces for guiding the light beam 154 which exits from the information display part 150. For this reason, it is difficult to give curvatures to the respective surfaces 158 and 157 and no aberration correction is effected at either of the entrance surface 158 and the exit surface 157.
The range of applications of either of the optical systems for optical pickups which are disclosed in, for example, Japanese Laid-Open Patent Application Nos. Hei 5-12704 and Hei 6-139612 is limited to the field of a detecting optical system, and neither of them satisfies the image forming performance required for, particularly, an image pickup apparatus which uses an area type of image pickup device, such as a CCD.
An object of the present invention is to provide a high-performance variable magnification optical system which includes a plurality of optical units two of which move relative to each other to realize variation of the magnification of the variable magnification optical system, the variable magnification optical system being capable of varying the magnification while varying the optical path length from an object to a final image plane with the final image forming plane spatially fixed, so that the thickness of the variable magnification optical system is small in spite of its wide angle of view and its entire length is short in a predetermined direction as well as its decentering aberration is fully corrected over the entire range of variation of magnification.
Another object of the present invention is to provide an image pickup apparatus using the aforesaid high-performance variable magnification optical system.
Another object of the present invention is to provide a variable magnification optical system having at least one of the following effects and advantages, and an image pickup apparatus employing such a variable magnification optical system.
Since a stop is arranged on the object side of the variable magnification optical system or in the vicinity of the first surface and an object image is formed by a plurality of times in the variable magnification optical system, the effective diameter and the thickness of the variable magnification optical system can be made small in spite of its wide angle of view.
Since each optical unit employs an optical element having a plurality of reflecting surfaces having appropriate refractive powers and the reflecting surfaces are arranged in a decentered manner, the optical path in the variable magnification optical system can be bent into a desired shape to reduce the entire length of the variable magnification optical system in a predetermined direction.
A plurality of optical elements which constitute the variable magnification optical system are each formed as a transparent body on which two refracting surfaces and a plurality of reflecting surfaces are integrally formed in such a manner that each of the reflecting surfaces is arranged in a decentered manner and is given an appropriate refractive power. Accordingly, the decentering aberration of the variable magnification optical system can be fully corrected over the entire range of variation of magnification.
Since each magnification varying optical unit employs an optical element which is formed as a transparent body on which two refracting surfaces and a plurality of curved or plane reflecting surfaces are integrally formed, not only is it possible to reduce the entire size of the variable magnification optical system, but it is also possible to solve the problem of excessively strict arrangement accuracy (assembly accuracy) which would have often been experienced with reflecting surfaces.
A variator optical unit which shows a largest amount of variation of magnification during a magnification varying operation is fixed, and an optical unit lying on the object side of the variator optical unit is moved to vary the magnification of the variable magnification optical system, so that an exit pupil on its telephoto side can be formed at a position more distant from an image plane than that on its wide-angle side. Accordingly, by appropriately setting the position of the exit pupil at the wide-angle end, it is possible to restrain occurrence of shading over the entire range of variation of magnification in an image pickup apparatus employing a solid-state image pickup device.
A variator optical unit which shows a largest amount of variation of magnification during a magnification varying operation is composed of an optical element having an entering reference axis and an exiting reference axis which differ from each other by 180xc2x0 in direction. The variator optical unit is fixed, and an optical unit lying on the object side of the variator optical unit is moved to vary the magnification of the variable magnification optical system, so that the distance of movement of a moving optical unit positioned on the image-plane side of the variator optical unit can be reduced.
To achieve the above objects, in accordance with one aspect of the present invention, there is provided a variable magnification optical system which comprises at least three optical units, the three optical units being a first moving optical unit, a fixed optical unit and a second moving optical unit which are arranged in that order in a propagation direction of light, a variation of magnification being effected by a relative movement between the first moving optical unit and the second moving optical unit, wherein if a ray which exits from an object and enters the variable magnification optical system, and passes through a center of a stop of the variable magnification optical system and reaches a center of a final image plane is represented as a reference axis ray; a reference axis ray which is incident on any surface of the variable magnification optical system or enters any of the optical units is represented as an entering reference axis of the aforesaid any surface or the aforesaid any optical unit; a reference axis ray which exits from the aforesaid any surface or the aforesaid any optical unit is represented as an exiting reference axis of the aforesaid any surface or the aforesaid any optical unit; a point at which the entering reference axis intersects with the aforesaid any surface is represented as a reference point; a direction in which the reference axis ray travels from an object side toward an image plane along the entering reference axis is represented as a direction of the entering reference axis; and a direction in which the reference axis ray travels from the object side toward the image plane along the exiting reference axis is represented as a direction of the exiting reference axis, the second moving optical unit has a cross-sectional shape which is asymmetrical in a plane which contains the reference axis, and a curved reflecting surface which is inclined with respect to the reference axis, and the direction of the entering reference axis and the direction of the exiting reference axis of the second moving optical unit are parallel to each other and differ from each other by 180xc2x0, the variable magnification optical system being arranged in such a manner that a final image is formed after an intermediate image is formed at least twice.
In the variable magnification optical system, the fixed optical unit is an optical unit having the largest ratio of (a lateral magnification at a wide-angle end) to (a lateral magnification at a telephoto end) of all the optical units.
In the variable magnification optical system, the first moving optical unit moves toward the fixed optical unit during a variation of magnification from a wide-angle end toward a telephoto end.
In the variable magnification optical system, the second moving optical unit includes an optical element which is formed as one transparent body on which two refracting surfaces and a plurality of internal curved reflecting surfaces are formed.
In the variable magnification optical system, the first moving optical unit includes an optical element which is formed as one transparent body on which two refracting surfaces and a plurality of internal curved reflecting surfaces inclined with respect to the reference axis are formed, the direction of the entering reference axis and the direction of the exiting reference axis of the optical element being parallel to and the same as each other.
In the variable magnification optical system, the first moving optical unit includes an optical element which is formed as one transparent body on which two refracting surfaces and a plurality of internal curved reflecting surfaces inclined with respect to the reference axis are formed, the direction of the entering reference axis and the direction of the exiting reference axis of the optical element being parallel to each other and different from each other by 180xc2x0.
In the variable magnification optical system, the first moving optical unit forms an intermediate image in its inside.
In the variable magnification optical system, the fixed optical unit includes an optical element which is formed as one transparent body on which two refracting surfaces and a plurality of internal curved reflecting surfaces inclined with respect to the reference axis are formed, the direction of the entering reference axis and the direction of the exiting reference axis of the optical element being parallel to and the same as each other.
In the variable magnification optical system, the fixed optical unit includes an optical element which is formed as one transparent body on which two refracting surfaces and a plurality of internal curved reflecting surfaces inclined with respect to the reference axis are formed, the direction of the entering reference axis and the direction of the exiting reference axis of the optical element being parallel to each other and different from each other by 180xc2x0.
In the variable magnification optical system, the fixed optical unit includes an optical element which is formed as one transparent body on which two refracting surfaces and a plurality of internal curved reflecting surfaces inclined with respect to the reference axis are formed, the exiting reference axis of the optical element being inclined with respect to the entering reference axis thereof.
In the variable magnification optical system, the stop is located on the object side of the first moving optical unit, the stop being fixed during the variation of magnification.
In accordance with another aspect of the present invention, there is provided a variable magnification optical system which comprises a fixed optical unit and a plurality of magnification varying optical units which are arranged in that order in a propagation direction of light, a variation of magnification being effected by a relative movement between the plurality of magnification varying optical units, wherein letting fi be a focal length of any magnification varying optical unit i and letting k be a number of times by which an on-axial light beam forms an intermediate image in the aforesaid any magnification varying optical unit i, the aforesaid any magnification varying optical unit i satisfies:
fixc2x7(xe2x88x921)k greater than 0 (k is an integer not less than 0),
and wherein if a ray which exits from an object and enters the variable magnification optical system, and passes through a center of a stop of the variable magnification optical system and reaches a center of a final image plane is represented as a reference axis ray; a reference axis ray which is incident on any surface of the variable magnification optical system or enters any of the optical units is represented as an entering reference axis of the aforesaid any surface or the aforesaid any optical unit; a reference axis ray which exits from the aforesaid any surface or the aforesaid any optical unit is represented as an exiting reference axis of the aforesaid any surface or the aforesaid any optical unit; a point at which the entering reference axis intersects with the aforesaid any surface is represented as a reference point; a direction in which the reference axis ray travels from an object side toward an image plane along the entering reference axis is represented as a direction of the entering reference axis; and a direction in which the reference axis ray travels from the object side toward the image plane along the exiting reference axis is represented as a direction of the entering reference axis, any of the magnification varying optical units includes at least one concave reflecting surface the entering and exiting reference axes of which are inclined with respect to a normal to the concave reflecting surface at the reference point thereof, the concave reflecting surface having a cross-sectional shape which is asymmetrical in a plane which contains the entering reference axis and the exiting reference axis.
The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments as well as numerical examples, taken in conjunction with the accompanying drawings.